1. Field of the Invention
The present invention relates generally to an NB-TDD CDMA (Narrow Band Time Division Duplexing Code Division Multiple Access) communication system, and in particular, to an apparatus and method for randomly assigning time slots of a sub-frame transmitting user data of a user equipment.
2. Description of the Related Art
In an NB-TDD CDMA mobile communication system, a radio network controller (RNC) assigns unique uplink and downlink channels to each user equipment (UE) through a physical channel, for radio communication with the UE. The UE then transmits frames to a Node B through the uplink physical channel assigned thereto, and receives frames transmitted from the Node B through the assigned downlink physical channel. In the NB-TDD CDMA mobile communication system, the physical channels transmit 100 frames per second. That is, each frame has a size (length) of 10 ms. Further, the frame is divided into two sub-frames, and each sub-frame is comprised of 7 time slots Ts(0)–Ts(6).
A structure of the sub-frames used in the NB-TDD CDMA mobile communication system will be described with reference to FIG. 1. In the following description, the 7 time slots Ts of the sub-frame are represented by Ts(0), Ts(1), Ts(2), Ts(3), Ts(4), Ts(5) and Ts(6) from the left hand side on a time axis.
Referring to FIG. 1, a sub-frame used in the NB-TDD CDMA mobile communication system has a length of 5 msec (=6400 chips). The sub-frame is comprised of 7 normal Ts, a downlink pilot time slot (DwPTS), and an uplink pilot time slot (UpPTS). Each Ts has a length of 864 chips, and the DWPTS and a guard period (GP) both have a length 96 chips. The UpPTS has a length of 160 chips. Further, the Ts are classified into uplink time slots and downlink time slots. A common NB-TDD CDMA mobile communication system fixedly designates Ts(0) as a downlink time slot and Ts(1) as an uplink time slot. In addition, a “switching point” indicating a boundary between the uplink time slot and the downlink time slot should occur two times in one sub-frame. The radio network controller determines the number of uplink time slots in the sub-frame for a specific channelization code, and assigns a time slot in response to a request from the UE. Once the UE is assigned the time slot, it continuously uses the assigned time slot until the call is ended. For example, if the UE is initially assigned Ts(5), it will transmit data using only Ts (5).
The term “frame” used in the NB-TDD CDMA mobile communication system refers to two consecutive sub-frames, and it has a length of 10 msec. Further, the term “multi-frame configuration (or structure)” refers to a mapping relationship between a specific Ts in several consecutive frames and a physical channel, a relationship that is repeated at set periods.
The multi-frame configuration will be described in detail. In the multi-frame configuration, a time slot #0 Ts(0) in each sub-frame can be mapped with a broadcast channel (BCH), a forward access channel (FACH) or a paging channel (PCH). A mapping relationship between Ts(0) and the above-stated physical channels of BCH, FACH and PCH can be predefined. The multi-frame configuration, as stated above, refers to a configuration where a frame is repeated in the common channels of BCH, FACH, PCH and PICH (Pilot Channel). However, since only the BCH is considered in the following description, only the configuration where the BCH is repeated will be defined as the multi-frame configuration.
In the NB-TDD CDMA mobile communication system, 200 sub-frames per second are arranged in the physical channel on the time axis at regular intervals, so that a signal transmitted by a specific UE is transmitted at the frequency of exactly 200 times per second. As a result, signals are concentrated on a frequency of 200 Hz belonging to an audible frequency band, causing an electromagnetic interference which may interrupt a voice (or circuit) call of the UE.